1. Field of the Invention
The present invention relates to a tapered slot antenna for use in high-frequency transmission, and more particularly, to a tapered slot antenna provided with a corrugated structure, having an improved directivity, for use in mobile communication equipment, small information terminals, etc.
2. Description of the Background
Tapered slot antennas have a number of potential applications as single elements and focal plane arrays. They have important advantages such as being light in weight, less expensively manufactured with printed circuit board techniques that are capable of accurate replication from unit to unit.
FIG. 10 is a perspective view for exemplifying such a known tapered slot antenna. Referring to FIG. 10, the tapered slot antenna 100 includes a substrate 120 and a metal layer 130 disposed thereon.
The substrate 120 is made of a dielectric material such as polyimide, having a thickness of 10 to 100 microns. In addition, the metal layer 130 preferably made of copper has a thickness of 2 to 20 microns, with a tapered portion 14 etched away to expose a portion of the dielectric substrate 120. This tapered portion 14 extends outward toward the aperture 15 of the slot antenna 100.
The tapered slot antenna radiates the electromagnetic wave to the direction parallel to the antenna plane (that is, along a slot line). Since the tapered slot antenna has a structure similar to that of a slot line without requirements for a grounded conductor on the backside thereof unlike the micro strip line, it can be integrated more easily with various components such as, for example, feeder lines and matching circuits in a uniplanar structure.
In addition, since the tapered slot antenna has a broad band of transmission frequency and a high antenna gain, it is adequately used in mobile communication equipment, small information terminal and other wireless communication apparatuses.
For the known tapered slot antenna 100, it is found that the D-plane cross-polarized component generally has a magnitude of as large as -6 dB to -9 dB. Referring to FIG. 11, the E- and H-planes of the tapered slot antenna are defined, respectively, as the planes which each includes the direction of radiation, and which is in parallel and in perpendicular to the surface plane of the antenna substrate 120. In addition, the D-plane is defined as that which makes a 45.degree. angle between both the E- and H-planes. Further, a cross-polarized wave is the electromagnetic wave which is polarized perpendicular to the polarization direction of the antenna.
A method of reducing the magnitude of a D-plane cross-polarized component is described by P. R. Acharya, J. Johansson and E. L. Kollberg, entitled SLOTLINE ANTENNA FOR MILLIMETER AND SUB-MILLIMETER WAVELENGTH, Proc. 20th Euro. Microwave Conf., Budapest, Hungary, pp. 353-358, September, 1990.
According to this publication, the magnitude of D-plane cross-polarized component can be reduced to -11 dB by fabricating a broken linearly tapered slot antenna (BLTSA). The tapered portion of this slot antenna has the shape composed of three compositional linear portions, each having consecutively different gradient.
This publication, however, does not detail further concerning the reasons why and how the D-plane cross-polarized component can be reduced. It is difficult, therefore, to fabricate such a slot antenna as above according to the description.
As indicated above, the D-plane cross-polarized component is large for the known tapered slot antenna. This gives rise to disadvantages for use in receiving more than one component, as exemplified hereinbelow: An antenna system is consisted of two tapered slot antennas with each slot line directed to the same direction and each antenna plane in perpendicular to each other to receive two polarized components separately. Because of the above-mentioned large intensity of D-plane cross-polarized component, one antenna in this system is sensitive not only to the component parallel to its own plane but also to the component perpendicular to its plane. Therefore, two polarized components can not be satisfactorily separated with the above antenna system.
In addition, the reasons for the aforementioned large magnitude of the D-plane cross-polarized component are yet to be clarified.